Transient free radicals in iron/oxygen reconstitution of mutant protein R2 Y122F. Possible participants in electron transfer chains in ribonucleotide reductase

Ferrous iron/oxygen reconstitution of the mutant R2 apoprotein Y122F leads to formation of a diferric center similar to that of the wild-type R2 protein of Escherichia coli ribonucleotide reductase. This reconstitution reaction requires two extra electrons, supplied or transferred by the protein matrix of R2. We observed several transient free radical species using stopped flow and freeze quench EPR and stopped flow UV-visible spectroscopy. Three of the radicals occur in the time window 0.1-2 s, i.e. concomitant with formation of the diferric site. They include a strongly iron-coupled radical (singlet EPR signal) observed only at < or = 77 K, a singlet EPR signal observed only at room temperature, and a radical at Tyr-356 (light absorption at 410 nm), an invariant residue proposed to be part of an electron transfer chain in catalysis. Three additional transient radicals species are observed in the time window 6 s to 20 min. Two of these are conclusively identified, by specific deuteration, as tryptophan radicals. Comparing side chain geometry and distance to the iron center with EPR characteristics of the radicals, we propose certain Trp residues in R2 as likely to harbor these transient radicals.

Residues important for radical stability in ribonucleotide reductase from Escherichia coli

The R2 protein of ribonucleotide reductase contains at the side chain of tyrosine 122 a stable free radical, which is essential for enzyme catalysis. The tyrosyl radical is buried in the protein matrix close to a dinuclear iron center and a cluster of three hydrophobic residues (Phe-208, Phe-212, and Ile-234) conserved throughout the R2 family. A key step in the generation of the tyrosyl radical is the activation of molecular oxygen at the iron center. It has been suggested that the hydrophobic cluster provides an inert binding pocket for molecular oxygen bound to the iron center and that it may play a role in directing the oxidative power of a highly reactive intermediate toward tyrosine 122. We have tested these hypotheses by constructing the following mutant R2 proteins:F208Y, F212Y, F212W, and I234N. The resulting mutant proteins all have the ability to form a tyrosine radical, which indicates that binding of molecular oxygen can occur. In the case of F208Y, the yield of tyrosyl radical is substantially lower than in the wild-type case. A competing reaction resulting in hydroxylation of Tyr-208 implies that the phenylalanine at position 208 may influence the choice of target for electron abstraction. The most prominent result is that all mutant proteins show impaired radical half-life; in three of the four mutants, the half-lives are several orders of magnitude shorter than that of the wild-type radical. This suggests that the major role of the hydrophobic pocket is to stabilize the tyrosyl radical. This hypothesis is corroborated by comparative studies of the environment of other naturally occurring tyrosyl radicals.

Generation of the glycyl radical of the anaerobic Escherichia coli ribonucleotide reductase requires a specific activating enzyme

The anaerobic ribonucleotide reductase from Escherichia coli contains a glycyl radical as part of its polypeptide structure. The radical is generated by an enzyme system present in E. coli. The reductase is coded for by the nrdD gene located at 96 min. Immediately downstream, we now find an open reading frame with the potential to code for a 17.5-kDa protein with sequence homology to a protein required for the generation of the glycyl radical of pyruvate formate lyase. The protein corresponding to this open reading frame is required for the generation of the glycyl radical of the anaerobic reductase and binds tightly to the reductase. The "activase" contains iron, required for activity. The general requirements for generation of a glycyl radical are identical for the reductase and pyruvate formate lyase. For the reductase, the requirement of an iron-containing activase suggests the possibility that the iron-sulfur cluster of the enzyme is not involved in radical generation but may participate directly in the reduction of the ribonucleotide.

Bacteriophage T4 gene 55.9 encodes an activity required for anaerobic ribonucleotide reduction

Bacteriophage T4 contains a phage-encoded anaerobic ribonucleoside triphosphate reductase, nrdD, previously named sunY. An open reading frame, 55.9, that resides downstream of the phage reductase was observed to have amino acid sequence similarity with the E. coli pyruvate formate-lyase (Pfl) activating enzyme. A stop codon was engineered into the cloned 55.9 gene and then recombined back into the phage genome. Phage-infected extracts that lack a functional 55.9 product have a 6-fold reduction in anaerobic ribonucleotide reductase activity and are unable to activate overexpressed T4 NrdD. Restoration of reductase activity was possible when 55.9- and nrdD- T4-infected Escherichia coli extracts were conjointly assayed. Comparing the anaerobic burst size of 55.9- infections to that of the parental phage indicates that anaerobic de novo synthesis of deoxyribonucleotides is nearly abolished in phage lacking the 55.9 product. We propose that T4 55.9 encodes an enzyme that activates T4 NrdD by generating a glycyl radical in the phage-encoded reductase. The homology between the Pfl activating enzyme and T4 55.9 product (in this communication renamed NrdG) in function as well as amino acid sequence is presumably a remnant of an ancient heritage between Pfl and the anaerobic ribonucleotide reductases.

The ribonucleotide reductase jigsaw puzzle: a large piece falls into place

The three-dimensional structure of ribonucleotide reductase protein R1 from Escherichia coli reveals a novel 10-stranded alpha/beta barrel fold. A long loop penetrates the center cavity to assemble the active site cysteine triad.

Intron-containing T4 bacteriophage gene sunY encodes an anaerobic ribonucleotide reductase

The function of the SunY protein, encoded by an intron-containing gene of bacteriophage T4, has remained hitherto unknown in contrast to the extensively studied self-splicing reaction of the SunY intron. Here we show that anaerobic T4 infections of Escherichia coli induce a ribonucleoside triphosphate reductase activity that is 10-30-fold higher than the bacterial host level of the corresponding enzyme. Inactivation of the T4 sunY gene (in this communication renamed nrdD) significantly decreased both the induced activity and the anaerobic production of phage, confirming the role of the T4 NrdD (SunY) protein as a phage-specific anaerobic ribonucleotide reductase. With the identification of the T4 nrdD (sunY) gene product as an anaerobic ribonucleotide reductase, all known bacteriophage introns are found to share the common and as yet unexplained property of residing within genes of DNA metabolism.

Tryptophan radicals formed by iron/oxygen reaction with Escherichia coli ribonucleotide reductase protein R2 mutant Y122F

The active state of the small subunit, protein R2, of ribonucleotide reductase is formed by the reaction of apoprotein with Fe2+ and O2, whereby the diferric site and a stable phenoxy free radical on a tyrosyl residue (Tyr122) is formed. The corresponding reaction was studied in the mutant Y122F R2. It leads to a normal iron site, but the reduction equivalent from Tyr122 now has to be supplied from elsewhere. EPR spectroscopy shows formation of several paramagnetic species on different time scales. Using apoprotein with deuterium-labeled tryptophan residues, at least two species could be assigned to tryptophan free radicals. This is the first EPR observation of relatively stable protein-linked tryptophan radicals at room temperature and at 77 K. These tryptophan radicals may be involved as redox intermediates in long range electron transfer within the protein structure.

The conserved serine 211 is essential for reduction of the dinuclear iron center in protein R2 of Escherichia coli ribonucleotide reductase

The R2 protein family of class I ribonucleotide reductases contains a highly conserved serine residue close to the essential tyrosyl radical and the dinuclear iron center. In order to test its physiological importance, we have engineered the Ser-211 of Escherichia coli R2 to an alanine and a cysteine residue. The three-dimensional structure of R2 S211A solved to 2.4-A resolution is virtually identical to the wild-type structure apart from the substituted residue. Both mutant proteins contain oxidized dinuclear iron and tyrosyl radical, and their specific enzyme activity per radical are comparable to that of the wild-type protein. In R2 S211A the stability of the tyrosyl radical is substantially decreased, probably caused by movement of Gln-80 into hydrogen bonding distance of Tyr-122. The major defect in R2 S211A, however, is the inability of its iron center to be reduced by enzymic or chemical means, a characteristic not found in R2 S211C. We propose that Ser-211 is needed as a proton donor/transporter during reduction of the iron center of R2, a reaction which in vivo precedes reconstitution of the tyrosyl radical. This offers a physiological explanation for the high conservation of a serine residue at this position in the R2 family.

Dioxygen is the source of the mu-oxo bridge in iron ribonucleotide reductase

The formation of the iron-radical cofactor in the R2 subunit of ribonucleotide reductase has been monitored by resonance Raman spectroscopy. The differrous cluster in reduced R2 functions as a tyrosine oxidase; it uses O2 to oxidize Tyr-122 to a stable radical and results in an oxo-bridged diferric cluster. The Phe-122 mutant produces an identical dinuclear iron center and provides a simplified model for O2 activation. Oxidation with 18O2 results in quantitative incorporation of 18O into the diferric cluster as evidenced by the 13-cm-1 downshift in the Fe-O-Fe stretching vibration at 500 cm-1. Thus, O2 must be coordinated to the diiron center during O-O bond cleavage. When the Phe-208 adjacent to the diferous cluster is mutated to Tyr, reaction with O2 results in its oxidation to dihydroxyphenylalanine (DOPA-208) and subsequent coordination to Fe as a catecholate ligand. The Fe-O/(catecholate) stretching modes at 512 and 592 cm-1 shift by -13 and -8 cm-1, respectively, when the oxidation is performed in H(2)18O. These isotope shifts indicate that the second oxygen atom of DOPA-208 originates from H2O rather than O2. Taken together, our results are consistent with a mu-1,1-peroxide and a high valent iron-oxo species as reaction intermediates. A common pathway for oxygen activation by the related iron-oxo enzymes methane monooxygenase and fatty acid desaturase is proposed.

Autocatalytic generation of dopa in the engineered protein R2 F208Y from Escherichia coli ribonucleotide reductase and crystal structure of the dopa-208 protein

The mutant form Phe-208-->Tyr of the R2 protein of Escherichia coli ribonucleotide reductase contains an intrinsic ferric-Dopa cofactor with characteristic absorption bands at 460 and ca. 700 nm [Ormö, M., de Maré, F., Regnström, K., Aberg, A., Sahlin, M., Ling, J., Loehr, T. M., Sanders-Loehr, J., & Sjöberg, B. M. (1992) J. Biol. Chem. 267, 8711-8714]. The three-dimensional structure of the mutant protein, solved to 2.5-A resolution, shows that the Dopa is localized to residue 208 and that it is a bidentate ligand of Fe1 of the binuclear iron center of protein R2. Nascent apoR2 F208Y, lacking metal ions, can be purified from overproducing cells grown in iron-depleted medium. ApoR2 F208Y is rapidly and quantitatively converted to the Dopa-208 form in vitro by addition of ferrous iron in the presence of oxygen. Other metal ions (Cu2+, Mn2+, Co2+) known to bind to the metal site of wild-type apoR2 do not generate a Dopa in apoR2 F208Y. The autocatalytic generation of Dopa does not require the presence of a tyrosine residue at position 122, the tyrosine which in a wild-type R2 protein acquires the catalytically essential tyrosyl radical. It is proposed that generation of Dopa initially follows the suggested reaction mechanism for tyrosyl radical generation in the wild-type protein and involves a ferryl intermediate, which in the case of the mutant R2 protein oxygenates Tyr 208. This autocatalytic metal-mediated reaction in the engineered R2 F208Y protein may serve as a model for formation of covalently bound quinones in other proteins.

Translation across the 5'-splice site interferes with autocatalytic splicing

The bacteriophage T4 nrdB gene, encoding the ribonucleotide reductase small subunit, contains a self-splicing group IA2 intron with an ochre codon in frame with the preceding exon sequence. The stop codon was changed to an amino acid codon and splicing efficiency was compared with that of the wild type in the presence and absence of translation. In vivo the mutant has a much lower efficiency for producing a mature transcript than the wild type. Also, the relative production of the full-length translation product is correspondingly lower in the mutant than in the wild type. These results confirm the importance of the stop codon, which spans the splice site of the nrdB intron. The occurrence of stop codons in 56 group I introns in protein-encoding genes was investigated. In 33 of those translation is terminated upstream of the first common elements of the catalytic core, of group I introns. In the rest translation is terminated in intron regions outside the heart of the catalytic core, with one exception. Our observations suggest that in situations where transcription and translation are coupled events there has been an evolutionary pressure to preserve stop codons in the 5'-region of these introns or to prevent translational termination from occurring in vital parts of the introns.

A possible glycine radical in anaerobic ribonucleotide reductase from Escherichia coli: nucleotide sequence of the cloned nrdD gene

During anaerobic growth of Escherichia coli an oxygen-sensitive ribonucleoside-triphosphate reductase, different from the aerobic ribonucleoside diphosphate-reductase (EC 1.17.4.1), produces the deoxyribonucleoside triphosphates required for DNA replication. The gene for the anaerobic enzyme has now been cloned and was found to contain a 2136-nucleotide coding region, corresponding to 712 amino acid residues, and an Fnr binding site 228 base pairs upstream of the initiator ATG. The deduced amino acid sequence shows 72% identity to a gene of coliphage T4, sunY, hitherto of unknown function, suggesting that the virus codes for its own anaerobic reductase. The location of an organic free radical formed during activation of the bacterial anaerobic reductase is proposed to be on Gly-681, since the pentapeptide RVCGY at positions 678-682 shows a striking similarity to the C-terminal sequence. RVSGY, of pyruvate formate-lyase. During activation of the anaerobically induced pyruvate formate-lyase, the glycine residue of the pentapeptide becomes an organic radical [Wagner, A. F. V., Frey, M., Neugebauer, F. A., Schäfer, W. & Knappe, J. (1992) Proc. Natl. Acad. Sci. USA 89, 996-1000]. The gene for the anaerobic reductase is located at a position around 96 min on the E. coli genomic map.

Escherichia coli ribonucleotide reductase. Radical susceptibility to hydroxyurea is dependent on the regulatory state of the enzyme

Ribonucleotide reductase catalyzes the reduction of ribonucleotides to their corresponding deoxyribonucleotides via a radical-mediated mechanism. The enzyme from Escherichia coli consists of the two non-identical proteins, R1 and R2, the latter of which contains the necessary free radical located to a tyrosine residue. The radical scavenger hydroxyurea was found to reduce the tyrosyl radical of R2 in a second-order reaction. The rate constant (0.50 M-1 s-1 at 25 degrees C) for this process was several orders of magnitude lower than the hydroxyurea-dependent reduction of free tyrosyl radicals in solution. This difference probably reflects the fact that the R2 tyrosyl radical is buried in the interior of the protein. Formation of the R1R2 complex changed the susceptibility of the radical to hydroxyurea in a manner that reflects the regulatory state of the holoenzyme. Furthermore, binding of substrate or product to the holoenzyme complex made the R2 radical at least 10 times more susceptible to inactivation by hydroxyurea than it was in the isolated R2 protein. One active site mutation in the R1 protein was shown to affect the sensitivity of the tyrosyl radical of R2 differently than wild type protein R1 does. Our results clearly show that the susceptibility of the tyrosyl radical in R2 to inactivation by hydroxyurea can be used as an efficient probe for the regulatory state of the holoenzyme complex.

Caracemide, a site-specific irreversible inhibitor of protein R1 of Escherichia coli ribonucleotide reductase

The anticancer drug caracemide, N-acetyl-N,O- di(methylcarbamoyl)hydroxylamine, and one of its degradation products, N-acetyl-O-methylcarbamoyl-hydroxylamine, were found to inhibit the enzyme ribonucleotide reductase of Escherichia coli by specific interaction with its larger component protein R1. No effect on the smaller protein R2 was observed. The effect of the degradation product was about 30 times lower than that of caracemide itself. The caracemide inactivation of R1 is irreversible, with an apparent second-order rate constant of 150 M-1 s-1. The R1R2 holoenzyme was approximately 30 times more sensitive to caracemide inactivation than the isolated R1 protein. The ribonucleotide reductase substrates were potent competitors of the caracemide inhibition, with a Kdiss for GDP binding to R1 of 80 microM. The reducing agent dithiothreitol was also found to be a potent competitor of caracemide inactivation. These results indicate that caracemide inactivates R1 by covalent modification at the substrate-binding site. By analogy with the known interaction between caracemide and acetylcholinesterase or choline acetyltransferase, we propose that the modification of R1 occurs at an activated cysteine or serine residue in the active site of the enzyme.

Site-directed mutagenesis and deletion of the carboxyl terminus of Escherichia coli ribonucleotide reductase protein R2. Effects on catalytic activity and subunit interaction

Ribonucleotide reductase from Escherichia coli consists of two dissociable, nonidentical homodimeric proteins called R1 and R2. The role of the C-terminal region of R2 in forming the R1R2 active complex has been studied. A heterodimeric R2 form with a full-length polypeptide chain and a truncated one missing the last 30 carboxyl-terminal residues was engineered by site-directed mutagenesis. Kinetic analysis of the binding of this protein to R1, compared with full-length or truncated homodimer, revealed that the C-terminal end of R2 accounts for all of its interactions with R1. The intrinsic dissociation constant of the heterodimeric R2 form, with only one contact to R1, 13 microM, is of the same magnitude as that obtained previously [Climent, I., Sjöberg, B.-M., & Huang, C. Y. (1991) Biochemistry 30, 5164-5171] for synthetic C-terminal peptides, 15-18 microM. We have also mutagenized the only two invariant residues localized at the C-terminal region of R2, glutamic acid-350 and tyrosine-356, to alanine. The binding of these mutant proteins to R1 remains tight, but their catalytic activity is severely affected. While E350A protein exhibits a low (240 times less active than the wild-type) but definitive activity, Y356A is completely inactive. A catalytic rather than structural role for these residues is discussed.

Engineering of the iron site in ribonucleotide reductase to a self-hydroxylating monooxygenase

Protein R2 of ribonucleotide reductase contains a dinuclear ferric iron center adjacent to a tyrosyl radical in the interior of the protein matrix. A patch of hydrophobic residues surrounds the iron-radical cofactor. Its importance during the oxidative generation of the iron-radical cofactor was investigated by site-directed mutagenesis of Phe-208 to tyrosine. The mutant protein R2 F208Y has prominent absorption bands at 460 and 720 nm reminiscent of those in ferric-catecholate complexes. Resonance Raman spectroscopy shows that the iron center of R2 F208Y contains a bidentate catechol ligand. The mechanism for generation of this protein-derived dihydroxyphenylalanine may be similar to the catalytic cycle of methane monooxygenase.

2'-Amino-2'-deoxyguanosine is a cofactor for self-splicing in group I catalytic RNA

In investigations on self-splicing in the group I intron of the pre-mRNA from the nrdB gene of bacteriophage T4 it was found that 2'-amino-2'-deoxyguanosine can replace guanosine as cofactor. This is the first guanosine-analogue with a modification in the 2'-position and substantial activity in a group I self-splicing reaction. The results suggest that the 2'-amino and 2'-hydroxy groups of the cosubstrates have some properties in common, which are important for binding as well as for catalysis.

Electron transfer associated with oxygen activation in the B2 protein of ribonucleotide reductase from Escherichia coli

Each of the two beta peptides which comprise the B2 protein of Escherichia coli ribonucleotide reductase (RRB2) possesses a nonheme dinuclear iron cluster and a tyrosine residue at position 122. The oxidized form of the protein contains all high spin ferric iron and 1.0-1.4 tyrosyl radicals per RRB2 protein. In order to define the stoichiometry of in vitro dioxygen reduction catalyzed by fully reduced RRB2 we have quantified the reactants and products in the aerobic addition of Fe(II) to metal-free RRB2apo utilizing an oxygraph to quantify oxygen consumption, electron paramagnetic resonance to measure tyrosine radical generation, and Mössbauer spectroscopy to determine the extent of iron oxidation. Our data indicate that 3.1 Fe(II) and 0.8 Tyr122 are oxidized per mol of O2 reduced. Mössbauer experiments indicate that less than 8% of the iron is bound as mononuclear high spin Fe(III). Further, the aerobic addition of substoichiometric amounts of 57Fe to RRB2apo consistently produces dinuclear clusters, rather than mononuclear Fe(III) species, providing the first direct spectroscopic evidence for the preferential formation of the dinuclear units at the active site. These stoichiometry studies were extended to include the phenylalanine mutant protein (Y122F)RRB2 and show that 3.9 mol-equivalents of Fe(II) are oxidized per mol of O2 consumed. Our stoichiometry data has led us to propose a model for dioxygen activation catalyzed by RRB2 which invokes electron transfer between iron clusters.

Carboxyl-terminal peptides as probes for Escherichia coli ribonucleotide reductase subunit interaction: kinetic analysis of inhibition studies

The active complex of Escherichia coli ribonucleotide reductase comprises two dissociable, nonidentical homodimeric proteins, B1 and B2. When B2 is the varied component, the reductase activity is competitively inhibited by synthetic peptides of varying lengths corresponding to the C-terminus of protein B2. This finding provides the first evidence that the C-terminal peptides and protein B2 share the same binding domain on protein B1. Our data also show that two molecules of peptide can bind to protein B1 with equal affinity. Similar inhibition constants (18 microM) were obtained for peptides containing the C-terminal 20, 30, and 37 residues. When the invariant residue Tyr 356 was omitted, a 2-fold decrease in peptide inhibitory ability was observed. A small peptide, lacking the last 11 residues, had virtually no inhibitory potency. These results, coupled with our previous observations that truncated protein B2, in which one or both polypeptide chains are missing approximately 24 C-terminal residues, had considerably lower or no affinity for B1, suggest that the C-terminal regions are the major determinants in the B1-B2 interaction. In the Appendix, two methods for treatment of kinetic situations pertinent to the ribonucleotide reductase system are presented. One method deals with the determination of kinetic parameters for two components present at comparable levels; the other is concerned with the differentiation of linear and nonlinear competitive inhibition involving the binding of two inhibitor molecules. Both methods should find application to other similar cases.

Three-dimensional model and molecular dynamics simulation of the active site of the self-splicing intervening sequence of the bacteriophage T4 nrdB messenger RNA

The secondary and 3D structure of the active site of the self-splicing T4 nrdB RNA has been modeled on a graphics workstation by use of the suggested 3D arrangement of the active site of the Tetrahymena IVS [Kim, S.H., & Cech, T.R. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 8788-8792] as a guideline. The initially obtained crude structure was then subjected to molecular mechanics energy minimization and molecular dynamics simulation to relax tensions. In this process the energy decreased considerably and gave a final structure that deviated by 3 A [root mean square (rms)] from the initial structure. The cofactor guanosine (and the competitive inhibitor arginine) was docked to a proposed [Michel, F., Hanna, M., Green, R., Bartel, D.P., & Szostak, J.W. (1989) Nature 342, 391-395] binding site, where it was found to fit rather well. A minor modification of the binding mode easily brought the O3' end of the guanosine within 2 A of the phosphodiester bond where the primary cleavage occurs.

An ultrafiltration assay for nucleotide binding to ribonucleotide reductase

Direct partition through ultrafiltration was applied to develop a method for the study of nucleotide binding to ribonucleotide reductase from Escherichia coli. The assay involved a 0.5- to 1-min centrifugation step where bound and unbound nucleotides are separated over an ultrafiltration membrane. No effects were seen due to hyperconcentration of protein at the membrane surface. The method was verified by measuring binding of dATP, ATP, dTTP, dGTP, and GDP at 25 and 4 degrees C with dissociation constants ranging from 0.1 to 80 microM. The results were in good agreement with earlier data obtained by other techniques and extend our knowledge in the case of ATP and dGTP binding at 25 degrees C.

Three-dimensional structure of the free radical protein of ribonucleotide reductase

The enzyme ribonucleotide reductase furnishes precursors for the DNA synthesis of all living cells. One of its constituents, the free radical protein, has an unusual alpha-helical structure. There are two iron centres that are about 25 A apart in the dimeric molecule. Tyrosine 122, which harbours the stable free radical necessary for the activity of ribonucleotide reductase, is buried inside the protein and is located 5 A from the closest iron atom.

Activation of the iron-containing B2 protein of ribonucleotide reductase by hydrogen peroxide

The active form of protein B2, the small subunit of ribonucleotide reductase, contains two dinuclear Fe(III) centers and a tyrosyl radical. The inactive metB2 form also contains the same diferric complexes but lacks the tyrosyl radical. We now demonstrate that incubation of metB2 with hydrogen peroxide generates the tyrosyl radical. The reaction is optimal at 5.5 nM hydrogen peroxide, with a maximum of 25-30% tyrosyl radical being formed after approximately 1.5 hr of incubation. The activation reaction is counteracted by a hydrogen peroxide-dependent reduction of the tyrosyl radical. It is likely that the generation of the radical proceeds via a ferryl intermediate, as in the proposed mechanisms for cytochrome P-450 and the peroxidases.

Modifications of the active center of T4 thioredoxin by site-directed mutagenesis

The active site sequence of T4 thioredoxin, Cys-Val-Tyr-Cys, has been modified in two positions to Cys-Gly-Pro-Cys to mimic that of Escherichia coli thioredoxin. The two point mutants Cys-Gly-Tyr-Cys and Cys-Val-Pro-Cys have also been constructed. The mutant proteins have similar reaction rates with T4 ribonucleotide reductase as has the wild-type T4 thioredoxin. Mutant T4 thioredoxins with Pro instead of Tyr at position 16 in the active site sequence have three to four times lower apparent KM with E. coli ribonucleotide reductase than wild-type T4 thioredoxin. The KM values for these mutant proteins which do not have Tyr in position 16 are thus closer to E. coli thioredoxin than to the wild-type T4 thioredoxin. The bulky tyrosine side chain probably prevents proper interactions to E. coli ribonucleotide reductase. Also the redox potentials of these two mutant thioredoxins are lower than that of the wild-type T4 thioredoxin and are thereby more similar to the redox potential of E. coli thioredoxin. Mutations in position 15 behave more or less like the wild-type protein. The kinetic parameters with E. coli thioredoxin reductase are similar for wild-type and mutant T4 thioredoxins except that the apparent kcat is lower for the mutant protein with Pro instead of Tyr in position 16. The active site sequence of T4 thioredoxin has also been changed to Cys-Pro-Tyr-Cys to mimic that of glutaredoxins. This change does not markedly alter the reaction rate of the mutant protein with T4 ribonucleotide reductase or E. coli thioredoxin reductase, but the redox potential is lower for this mutant protein than for wild-type T4 thioredoxin.